Single focal length lens system and image pickup apparatus using the same

ABSTRACT

A single focal length lens system includes a front lens unit and a rear lens unit, and does not include any other lens unit. The front lens unit includes a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power. Each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and the first lens unit and the second lens unit do not move all the time. An aperture stop is disposed on the image side of the second lens unit. Any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and the following conditional expression (1) is satisfied:
 
0.06&lt;| f   G3   /f |&lt;0.4  (1)

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based upon and claims the benefit of priority from the prior Japanese patent Application No. 2013-239785 filed on Nov. 20, 2013; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a single focal length lens system, and an image pickup apparatus using the same.

Description of the Related Art

In a photography in which, a telephoto lens or an ultra-telephoto lens (hereinafter, appropriately let to be telephoto lens) is used, an effect of drawing a distant object or a small object in front of an eye of a photographer is achieved. Therefore, the telephoto lens has widely been used in photography of various scenes such as photography of sport scenes, photography of wild animals such as wild birds, and photography of astronomical bodies.

As a telephoto lens to be used for photography of such scenes, telephoto lenses disclosed in Japanese Patent Application Laid-open Publication Nos. 2008-145584 and Hei 11-160617 are available.

In the photography of abovementioned scenes, relative merits of mobility of an image pickup apparatus become important. Here, the mobility refers to an ease of carrying, a stability at the time of hand-held photography, and a rapidity of focusing speed. For making the mobility of an apparatus superior, an optical system having a small size and light weight is desirable. Moreover, a feature that an optical system is capable of focusing an object rapidly is an important feature that decides the relative merits of mobility.

SUMMARY OF THE INVENTION

A single focal length lens system according to the present invention includes in order from an object side to an image side along an optical axis,

a front lens unit, and

a rear lens unit, wherein

the single focal length lens system does not include any other lens unit on the optical axis, and

the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and

each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and

the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and

an aperture stop is disposed on the image side of the second lens unit, and

any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and

the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and

the following conditional expression (1) is satisfied: 0.06<|_(G3) /f|<0.4  (1)

where,

f_(G3) denotes a focal length of the third lens unit, and

f denotes a focal length of the single focal length lens system at the time of focusing at an object at infinity.

Moreover, a single focal length lens system according to the present invention includes in order from an object side to an image side along an optical axis,

a front lens unit, and

a rear lens unit, wherein

the single focal length lens system does not include any other lens unit on the optical axis, and

the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and

each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and

the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and

an aperture stop is disposed on the image side of the second lens unit, and

any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and

the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and

the following conditional expression (2) is satisfied: 0.03<f _(b) /f<0.2  (2)

where,

f_(b) denotes a distance along the optical axis from an image-side surface of a lens nearest to an image, of the single focal length lens system up to an image plane, at the time of focusing at an object at infinity, and

f denotes a focal length of the single focal length lens system at the time of focusing at an object at infinity.

Moreover, a single focal length lens system according to the present invention includes in order from an object side to an image side along an optical axis,

a front lens unit, and

a rear lens unit, wherein

the single focal length lens system does not include any other lens unit on the optical axis, and

the front lens unit included in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and

each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and

the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and

an aperture stop is disposed on the image side of the second lens unit, and

any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and

the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and

the following conditional expressions (3) and (4) are satisfied: 1<ν_(G2nMAX)−ν_(G2pMIN)<40  (3) 37<ν_(G2nMAX)<55  (4)

where,

ν_(G2nMAX) denotes a maximum Abbe number from among Abbe numbers for negative lenses in the second lens unit, and

ν_(G2pMIN) denotes a minimum Abbe number from among Abbe numbers for positive lenses in the second lens unit.

Moreover, a single focal length lens system according to the present invention includes in order from an object side to an image side along an optical axis,

a front lens unit, and

a rear lens unit, wherein

the single focal length lens system does not include any other lens unit on the optical axis, and

the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and

each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and

the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and

an aperture stop is disposed on the image side of the second lens unit, and

any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and

the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and

the following conditional expression (5) is satisfied: 0.6<L _(TL) /f<0.88  (5)

where,

L_(TL) denotes a distance along the optical axis from an object-side surface of a lens nearest to the object, of the single focal length lens system up to an image plane, at the time of focusing at an object at infinity, and

f denotes a focal length of the single focal length lens system at the time of focusing at an object at infinity.

Moreover, an image pickup apparatus according to the present invention includes,

an optical system, and

an image pickup element which has an image pickup surface, and which converts an image formed on the image pickup surface by the optical system, to an electric signal, wherein

the optical system is any one of the abovementioned single focal length lens systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are lens cross-sectional views at the time of focusing at an object at infinity of a single focal length lens system according to the present invention, where, FIG. 1A is a lens cross-sectional view of a single focal length lens system according to an example 1, and FIG. 1B is a lens cross-sectional view of a single focal length lens system according to an example 2;

FIG. 2A and FIG. 2B are lens cross-sectional views at the time of focusing at an object at infinity of a single focal length lens system according to the present invention, where, FIG. 2A is a lens cross-sectional view of a single focal length lens system according to an example 3, and FIG. 2B is a lens cross-sectional view of a single focal length lens system according to an example 4;

FIG. 3A, FIG. 3B, FIG. 3C, and FIG. 3D are aberration diagrams at the time of focusing at an object at infinity of the single focal length lens system according to the example 1, and FIG. 3E, FIG. 3F, FIG. 3G, and FIG. 3H are aberration diagrams at the time of focusing at an object at a close distance of the single focal length lens system according to the example 1;

FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D are aberration diagrams at the time of focusing at an object at infinity of the single focal length lens system according to the example 2, and FIG. 4E, FIG. 4F, FIG. 4G, and FIG. 4H are aberration diagrams at the time of focusing at an object at a close distance of the single focal length lens system according to the example 2;

FIG. 5A, FIG. 5B, FIG. 5C, and FIG. 5D are aberration diagrams at the time of focusing at an object at infinity of the single focal length lens system according to the example 3, and FIG. 5E, FIG. 5F, FIG. 5G, and FIG. 5H are aberration diagrams at the time of focusing at an object at a close distance of the single focal length lens system according to the example 3;

FIG. 6A, FIG. 6B, FIG. 6C, and FIG. 6D are aberration diagrams at the time of focusing at an object at infinity of the single focal length lens system according to the example 4, and FIG. 6E, FIG. 6F, FIG. 6G, and FIG. 6H are aberration diagrams at the time of focusing at an object at a close distance of the single focal length lens system according to the example 4;

FIG. 7 is a cross-sectional view of a digital camera in which the single focal length lens system according to the example 1 is incorporated;

FIG. 8 is a front perspective view of the digital camera;

FIG. 9 is a rear perspective view of the digital camera; and

FIG. 10 is a schematic block diagram of an internal circuit of main components of the digital camera.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments and examples of a single focal length lens system, and an image pickup apparatus using the same according to the present invention will be described below in detail by referring to the accompanying diagrams. However, the present invention is not restricted to the embodiments and the examples described below.

Prior to the describing the single focal length lens system according to the present embodiment, a basic arrangement of the single focal length lens system according to the present embodiment will be described below. In the following description, the ‘single focal length lens system’ is appropriately referred to as ‘lens system’.

In the basic arrangement, the lens system includes in order from an object side to an image side along an optical axis, a front lens unit and a rear lens unit, and the single focal length lens system does not include any other lens unit on the optical axis, and the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and an aperture stop is disposed on the image side of the second lens unit, and any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance.

As aforementioned, the lens system includes in order from the object side to the image side, the front lens unit, and the rear lens unit. Here, the lens system does not include any other lens unit on the optical axis, or in other words, the lens system does not include a lens unit other than the front lens unit and the rear lens unit. Therefore, the lens units disposed on the optical axis are the front lens unit and the rear lens unit only. A cover glass and various optical filters, not being lens units, may be disposed on the optical axis (in an optical path).

For shortening the overall length of the lens system, it is necessary to arrange the lens system to include a telephoto arrangement, as well as to enhance an effect attributed to the telephoto arrangement (hereinafter, appropriately referred to as ‘effect due to the telephoto arrangement’). For enhancing the effect due to the telephoto arrangement, it is preferable to dispose a lens unit having a positive refractive power nearest to the object, and to dispose a lens unit having a negative refractive power on the image side thereof. By making such an arrangement, correction of a spherical aberration, a coma, and an astigmatism becomes possible mainly, while enhancing the effect due to the telephoto arrangement.

Here, the front lens unit includes in order from the object side to the image side, the lens unit having a positive refractive power and the lens unit having a negative refractive power. Therefore, even in the lens system according to the present embodiment, since an effect due to a telephoto arrangement is enhanced, it is possible to shorten the overall length of the lens system. Moreover, by making such an arrangement, it is possible to correct mainly a spherical aberration, a coma, and an astigmatism favorably, while enhancing the effect due to the telephoto arrangement.

More specifically, the lens unit having a positive refractive power is let to be at least the first lens unit, and the lens units having a negative refractive power are let to be at least the second lens unit and the third lens unit. In such manner, by making the lens unit having a negative refractive power of at least two lens units, it is possible to share the correction of the spherical aberration, the coma, and the astigmatism in the front lens unit between the second lens unit and the third lens unit. Therefore, it is possible to correct these aberrations favorably in the front lens unit.

Moreover, by letting the second lens unit and the third lens unit to have a negative refractive power, it is possible to make the negative refractive power in the front lens unit large without worsening these aberrations. Accordingly, since it is possible to enhance further the effect due to the telephoto arrangement, further shortening of the overall length of the optical system becomes possible.

Moreover, each of the first lens unit and the second lens unit includes the positive lens and the negative lens.

For enhancing the effect due to the telephoto arrangement in the front lens unit, it is preferable that each of the first lens unit and the second lens unit includes at least the positive lens and the negative lens. By making such an arrangement, it is possible to reduce the occurrence of aberrations such as the spherical aberration, the coma, the astigmatism, and the chromatic aberration in each of the first lens unit and the second lens unit. As a result, it is possible to achieve a favorable imaging performance even while small-sizing the lens system.

Moreover, the first lens unit and the second lens unit do not move, and the positions thereof are fixed all the time. In other words, in the front lens unit, the first lens unit and the second lens unit are stationary all the time.

A reason for making such an arrangement will be described below. When an attempt is made to carry out focusing, zooming, or image-motion correction by a lens (lens unit) in the front lens unit, the lens in the front lens unit is to be moved. However, as aforementioned, the effect due to the telephoto arrangement has been enhanced in the front lens unit. Therefore, if from among lenses in the front lens unit, particularly a lens positioned on the image side is moved, a large spherical aberration, coma, and astigmatism occur (fluctuate) with the movement of the lens.

Moreover, since a light ray height is high in the front lens unit, an amount of these aberrations occurring in the front lens unit becomes large as compared to an amount of these aberrations occurring in the rear lens unit. For such reason, if from among the lenses in the front lens unit, particularly the lens positioned on the image side is moved, an imaging performance is degraded. For preventing degradation of the imaging performance, it is necessary to reduce the occurrence (fluctuation) of these aberrations.

In such manner, if an attempt is made to move a lens in the front lens unit, there arises a need of achieving both of a maintenance of the effect due to the telephoto arrangement, and correction of aberrations in the front lens unit. Therefore, by letting the positions of the first lens unit and the second lens unit to be fixed all the time, a proportion of load of aberration correction on the front lens unit is reduced. As a result, it becomes possible to enhance the effect due to the telephoto arrangement in the front lens unit.

Moreover, the aperture stop is disposed on the image side of the second lens unit. By making such an arrangement, it is possible to make a lens diameter small in a lens unit on the image side of the aperture stop. Particularly, it is preferable to make the positive refractive power of the first lens unit and the negative refractive power of the second lens unit large, and to dispose the aperture stop on the image side of the second lens unit. By making such an arrangement, it is possible to make the lens diameter in the lens unit on the image side of the aperture stop further smaller.

Moreover, any one of the lens units disposed on the image side of the second lens unit islet to be the focusing lens unit, and the focusing lens unit is moved along the optical axis at the time of focusing from an object at infinity to an object at a close distance. By making such an arrangement, it is possible to carry out focusing. Moreover, in a case in which, the focusing lens unit is disposed on the image side of the aperture stop, it is possible to make the lens diameter in the lens system on the image side of the aperture stop small as aforementioned. As a result of this, it is possible to make a diameter of the focusing lens unit small. Moreover, since it is possible to make the diameter of the focusing lens unit small, it becomes possible to make a diameter of focusing unit (an arrangement including the focusing lens and a moving mechanism) as well, extremely small. It is preferable that there is at least one focusing lens unit, and there may be a plurality of focusing units as well.

A single focal length lens system according to a first embodiment will be described below. The single focal length lens system according to the first embodiment includes in order from an object side to an image side along an optical axis, a front lens unit and a rear lens unit, and the single focal length lens system does not include any other lens unit on the optical axis, and the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and an aperture stop is disposed on the image side of the second lens unit, and any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and the following conditional expression (1) is satisfied: 0.06<|f _(G3) /f|<0.4  (1)

where,

f_(G3) denotes a focal length of the third lens unit, and

f denotes a focal length of the single focal length lens system at the time of focusing at an object at infinity.

The lens system according to the first embodiment has the abovementioned basic arrangement, and also conditional expression (1) is satisfied. Since the technical significance of the basic arrangement has already been explained, description thereof is omitted here.

When falling below a lower limit value of conditional expression (1), since the refractive power of the third lens unit becomes excessively large, mainly the spherical aberration occurs largely. In this case, a proportion of load of correction of the spherical aberration on the rear lens unit increases. As a result of this, the number of lenses in the rear lens unit increases. In such manner, falling below the lower limit value of conditional expression (1) leads to an increase in a weight of the overall optical system.

When exceeding an upper limit value of conditional expression (1), a proportion of load of correction of the astigmatism on the second lens unit increased. In this case, since the negative refractive power of the second lens unit becomes large, a lens diameter in the second lens unit becomes large. As a result of this, a weight of the second lens unit increases. Moreover, the lens diameter in the second lens unit is larger as compared to a lens diameter in the third lens unit. Therefore, making a size of the second lens unit large is not preferable as it increases the weight of the overall optical system as compared to the weight of the overall optical system when a size of the third lens unit is made large.

A single focal length lens system according to a second embodiment includes in order from an object side to an image side along an optical axis, a front lens unit and a rear lens unit, and the single focal length lens system does not include any other lens unit on the optical axis, and the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and an aperture stop is disposed on the image side of the second lens unit, and any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and the following conditional expression (2) is satisfied: 0.03<f _(b) /f<0.2  (2)

where,

f_(b) denotes a distance along the optical axis from an image-side surface of a lens nearest to an image, of the single focal length lens system up to an image plane, at the time of focusing at an object at infinity, and

f denotes a focal length of the single focal length lens system at the time of focusing at an object at infinity.

The lens system according to the second embodiment has the abovementioned basic arrangement, and also conditional expression (2) is satisfied. Since the technical significance of the basic arrangement has already been explained, description thereof is omitted here.

In a case of facilitating shortening of the overall length of the optical system by enhancing the effect due to the telephoto arrangement, it is desirable that the front lens unit has a positive refractive power. Moreover, in the rear lens unit, it is necessary to correct the spherical aberration and the coma remained in the front lens unit. An effect of correction of these aberrations depends on a distance between refracting surfaces in the rear lens unit. Therefore, it is necessary to secure appropriately the distance between the refracting surfaces for enhancing the effect of correction of aberrations.

When falling below a lower limit value of conditional expression (2), since a back focus becomes short, a diameter of a lens disposed near an image plane becomes large. Therefore, falling below the lower limit value of conditional expression (2) is not preferable for making the optical system light-weight.

When exceeding an upper limit value of conditional expression (2), it is not possible to secure sufficiently the distance between the refracting surfaces in the rear lens unit. Therefore, when an attempt is made to shorten the overall length of the optical system, an amount of the spherical aberration and the coma that occur, increases. As a result of this, a favorable imaging performance is not achieved.

A single focal length lens system according to a third embodiment includes in order from an object side to an image side along an optical axis, a front lens unit and a rear lens unit, and the single focal length lens system does not include any other lens unit on the optical axis, and the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and an aperture stop is disposed on the image side of the second lens unit, and any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and the following conditional expressions (3) and (4) are satisfied: 1<ν_(G2nMAX)−ν_(G2pMIN)<40  (3) 37<ν_(G2nMAX)<55  (4)

where,

ν_(G2nMAX) denotes a maximum Abbe number from among Abbe numbers for negative lenses in the second lens unit, and

ν_(G2pMIN) denotes a minimum Abbe number from among Abbe numbers for positive lenses in the second lens unit.

The lens system according to the third embodiment has the abovementioned basic arrangement, and also conditional expressions (3) and (4) are satisfied. Since the technical significance of the basic arrangement has already been explained, description thereof is omitted here.

In the front lens unit, it is desirable to impart a positive refractive power to the overall front lens unit while enhancing an effect due to the telephoto arrangement. Accordingly, since it is possible to reduce a proportion of load of the refractive power in the rear lens unit, it is possible to shorten the overall length of the lens system as well as to reduce an aberration which occurs in the rear lens unit. As a result, it is possible to secure a favorable imaging performance.

When falling below a lower limit value of conditional expression (3), a longitudinal chromatic aberration and a chromatic aberration of magnification which occur in the second lens unit become large. In this case, a proportion of load of the chromatic aberration on the third lens unit increases. However, since the third lens unit cannot bear this increased load, it becomes difficult to correct sufficiently the chromatic aberration in the front lens unit. Moreover, it is possible to correct the chromatic aberration in the third lens unit but, in this case, it leads to an increase in the number of lenses in the third lens unit.

When exceeding an upper limit value of conditional expression (3), correction of the longitudinal chromatic aberration or the chromatic aberration of magnification in the second lens unit becomes excessive. Moreover, it becomes difficult to make a proportion of load of the refractive power of the second lens unit and the third lens unit appropriate. Therefore, it becomes difficult to suppress the occurrence of the spherical aberration, the coma, and the astigmatism in the front lens unit while shortening the overall length of the lens system.

Moreover, an amount of aberration remained in the front lens unit becomes large. It is necessary to correct the remained aberration in the rear lens unit. However, if the amount of the aberration remained in the front lens unit is large, a proportion of load of aberration correction on the rear lens unit increases. Therefore, correction of the remained aberration in the rear lens unit leads to an increase in the number of lenses in the rear lens unit.

When falling below a lower limit value of conditional expression (4), since a dispersion by the negative lens becomes excessively large, it becomes difficult to correct the longitudinal chromatic aberration and the chromatic aberration of magnification in a balanced manner. When exceeding an upper limit value of conditional expression (4), since correction of the chromatic aberration tends to be excessive, it becomes difficult to correct the chromatic aberration favorably.

A single focal length lens system according to a fourth embodiment includes in order from an object side to an image side along an optical axis, a front lens unit and a rear lens unit, and the single focal length lens system does not include any other lens unit on the optical axis, and the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and an aperture stop is disposed on the image side of the second lens unit, and any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and the following conditional expression (5) is satisfied: 0.6<L _(TL) /f<0.88  (5)

where,

L_(TL) denotes a distance along the optical axis from an object-side surface of a lens nearest to the object, of the single focal length lens system up to an image plane, at the time of focusing at an object at infinity, and

f denotes a focal length of the single focal length lens system at the time of focusing at an object at infinity.

The lens system according to the fourth embodiment has the abovementioned basic arrangement, and also conditional expression (5) is satisfied. Since the technical significance of the basic arrangement has already been explained, description thereof is omitted here.

When falling below a lower limit value of conditional expression (5), the positive refractive power of the first lens unit becomes excessively large. Here, the first lens unit is a lens unit in which the lens diameter is the maximum in the optical system. Therefore, the increase in the refractive power of the first lens unit leads to an increase in the number of lenses in the first lens unit. As a result of this, making the lens system light-weight is not facilitated.

When exceeding an upper limit value of conditional expression (5), the positive refractive power of the first lens unit becomes excessively small. In this case, it becomes difficult to enhance the effect due to the telephoto arrangement in the front lens unit. Therefore, intrinsic merits which can be achieved by the telephoto arrangement are receded. Moreover, the merit of sharing the negative refractive power between the second lens unit and the third lens unit is also receded. Forming the lens unit having a negative refractive power by two lens units, by contrast, leads to an increase in the weight of the optical system.

Moreover, in the single focal length lens systems from the single focal length lens system according to the first embodiment to the single focal length lens system according to the fourth embodiment (hereinafter, appropriately referred to as the lens system according to the present embodiment), it is preferable that the third lens unit is the focusing lens unit.

In the lens system according to the present embodiment, it is possible to reduce an amount of aberration that occurs, while making the refractive power of the third lens unit large. In this case, even when the focusing is carried out by the third lens unit, it is possible to suppress fluctuation in aberration due to the movement of the third lens unit. Therefore, even when the focusing is carried out by the third lens unit, it is possible to maintain a favorable imaging performance.

Moreover, in the lens system according to the present embodiment, it is preferable that the third lens unit is the only focusing lens unit.

The technical significance of letting the third lens unit to be the focusing lens unit is as mentioned above.

By letting the third lens unit to be the only focusing lens unit, it is possible to minimize the number of lens units to be moved. Therefore, making the focusing unit small-sized and light-weight is facilitated.

Moreover, in the lens system according to the present embodiment, it is preferable that the following conditional expression (6) is satisfied: 1.5<f _(G2) /f _(G3)<6.0  (6)

where,

f_(G2) denotes a focal length of the second lens unit, and

f_(G3) denotes a focal length of the third lens unit.

When falling below a lower limit value of conditional expression (6), the refractive power of the third lens unit becomes excessively small. In this case, if an attempt is made to carry out focusing by the third lens unit, an amount of movement of the third lens unit increases. When an attempt is made to secure a space which is necessary for the movement of the third lens unit, shortening the overall length of the lens systems becomes difficult.

When exceeding an upper limit value of conditional expression (6) an amount of the spherical aberration, the coma, and the astigmatism that occur in the front lens unit becomes large. In a case of correcting these aberrations by a lens unit in the front lens unit, a proportion of load of aberration correction on the third lens unit also increases. Therefore, when focusing has been carried out by the third lens unit, it becomes difficult to maintain a favorable imaging performance.

Moreover, in the lens system according to the present embodiment, it is preferable that the following conditional expression (3) is satisfied: 1<ν_(G2nMAX)−ν_(G2pMIN)<40  (3)

where,

ν_(G2nMAX) denotes a maximum Abbe number from among Abbe numbers for negative lenses in the second lens unit, and

ν_(G2pMIN) denotes a minimum Abbe number from among Abbe numbers for positive lenses in the second lens unit.

Since the technical significance of conditional expression (3) is as already been explained, description thereof is omitted here.

Moreover, in the lens system according to the present embodiment, it is preferable that the following conditional expression (7) is satisfied: −0.4<f _(fo) /f<−0.06  (7)

where,

f_(fo) denotes a focal length of the focusing lens unit, and

f denotes the focal length of the single focal length lens system at the time of focusing at an object at infinity.

When exceeding an upper limit value of conditional expression (7), the refractive power of the focusing lens unit becomes excessively large. In this case, a proportion of load of aberration correction on the focusing lens unit becomes excessively large. Therefore, it becomes difficult to maintain a favorable imaging performance in an overall area of a focusing range.

When falling below a lower limit value of conditional expression (7), an amount of movement of the focusing lens unit at the time of focusing increases. Therefore, when an attempt is made to secure a space which is necessary for the movement of the focusing lens unit, shortening the overall length of the lens system becomes difficult.

Moreover, in the lens system according to the present embodiment, it is preferable that the front lens unit has a positive refractive power as a whole.

In the front lens unit, it is desirable to impart a positive refractive power to the overall front lens unit while enhancing an effect due to the telephoto arrangement. Accordingly, since it is possible to reduce a proportion of load of the refractive power in the rear lens unit, it is possible to shorten the overall length of the lens system as well as to reduce an aberration which occurs in the rear lens unit. As a result, it is possible to secure a favorable imaging performance.

Moreover, in the lens system according to the present embodiment, it is preferable that the focusing lens unit has a negative refractive power.

By letting the refractive power of the focusing lens unit to be a negative refractive power, it is possible to enhance an effect of bringing a principal point closer to the object side. As a result, shortening the overall length of the lens system becomes easier.

Moreover, in the lens unit according to the present embodiment, it is preferable that the first lens unit includes a 1-1^(th) sub lens unit on the object side, and a 1-2^(th) sub lens unit on the image side, and an air space on an optical axis between the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit is the longest in the first lens unit, and each of the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit has a positive refractive power, and each of the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit includes a positive lens and a negative lens, and the following conditional expression (8) is satisfied: 0.35<D _(G1) /f _(G1)<1.3  (8)

where,

D_(G1) denotes a distance on the optical axis from a lens surface nearest to an object of the first lens unit up to a lens surface nearest to an image of the first lens unit, and

f_(G1) denotes the focal length of the first lens unit.

For enhancing the effect due to the telephoto arrangement in the front lens unit, it is necessary to make the positive refractive power of the first lens unit large. However, when such an arrangement is made, a curve of the spherical aberration (an amount of the spherical aberration that occurs) becomes large. Therefore, the positive refractive power of the first lens unit is divided between the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit. By making such an arrangement, even if the refractive power of the first lens unit is made large, it is possible to make an arrangement of the first lens unit an arrangement in which, the curve of the spherical aberration is small. Furthermore, an arrangement is let to be such that each of the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit includes the positive lens and the negative lens. By making such an arrangement, it is possible to reduce a curve of the chromatic aberration (an amount of the chromatic aberration that occurs) in the first lens unit.

When falling below a lower limit value of conditional expression (8), a lens diameter in the 1-2^(th) sub lens unit becomes large. Therefore, making the overall lens system light-weight cannot be facilitated. Furthermore, since the curve of the spherical aberration increases, securing a favorable imaging performance becomes difficult.

When exceeding an upper limit value of conditional expression (8), shortening the overall length of the lens system becomes difficult.

Moreover, in the lens system according to the present embodiment, it is preferable that the first lens unit includes a 1-1^(th) sub lens unit on the object side, and a 1-2^(th) sub lens unit on the image side, and an air space on an optical axis between the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit is the longest in the first lens unit, and each of the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit has a positive refractive power, and each of the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit includes a positive lens and a negative lens, and the following conditional expression (9) is satisfied: 0.1<D _(G112) /f _(G1)<0.6  (9)

where,

D_(G112) denotes a distance on the optical axis between the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit, and

f_(G1) denotes the focal length of the first lens unit.

For enhancing the effect due to the telephoto arrangement in the front lens unit, it is necessary to make the positive refractive power of the first lens unit large. However, when such an arrangement is made, the curve of the spherical aberration (an amount of the spherical aberration that occurs) becomes large. Therefore, the positive refractive power of the first lens unit is divided between the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit. By making such an arrangement, even if the refractive power of the first lens unit is made large, it is possible to make the arrangement of the first lens unit an arrangement in which, the curve of the spherical aberration is small. Furthermore, an arrangement is let to be such that each of the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit includes the positive lens and the negative lens. By making such an arrangement, it is possible to reduce the curve of the chromatic aberration (an amount of the chromatic aberration that occurs) in the first lens unit.

When falling below a lower limit value of conditional expression (9), a lens diameter in the 1-2^(th) sub lens unit becomes large. Therefore, making the overall lens system light-weight cannot be facilitated. Furthermore, since the curve of the spherical aberration increases, securing a favorable imaging performance becomes difficult.

When exceeding an upper limit value of conditional expression (9), shortening the overall length of the lens system becomes difficult.

Moreover, in the lens system according to the present embodiment, it is preferable that the rear lens unit includes an image-motion correcting lens unit, and the image-motion correcting lens unit moves in a direction different from a direction of the optical axis to reduce an image motion due to shaking of the single focal length lens system.

By shifting a lens unit having a refractive power, it is possible to correct a shift in an imaging position caused due to camera shake. Here, shifting the lens unit means moving the lens unit in the direction different from the direction of the optical axis to reduce an image motion due to shaking of the lens system. It is desirable that the lens unit which is to be shifted is small-sized and light-weight.

In a lens system having a telephoto arrangement, the rear lens unit becomes a lens unit having the smallest lens diameter. Therefore, since the lens system according to the present embodiment adopts the telephoto arrangement, it is preferable to dispose the lens unit to be shifted, or in other words, the image-motion correcting lens unit, in the rear lens unit. By making such an arrangement, since it is possible to make the image-motion correcting lens unit small-sized and light-weight, it is possible to improve a response of the image-motion correction.

Moreover, in the lens system according to the present embodiment, it is preferable that the rear lens unit includes a first predetermined lens unit which is disposed immediately before the image-motion correcting lens unit, and the first predetermined lens unit has a refractive power having a sign different from a sign of a refractive power of the image-motion correcting lens unit.

By making such an arrangement, it is possible to make the refractive power of the image-motion correcting lens unit large. As a result, it is possible to make an amount of shift in the imaging position large with respect to an amount of shift of the image-motion correcting lens unit. Accordingly, it is possible to carry out image-motion correction of higher accuracy with a small amount of shift.

Moreover, in the lens system according to the present embodiment, it is preferable that the rear lens unit includes a second predetermined lens unit which is disposed immediately after the image-motion correcting lens unit, and the second predetermined lens unit has a refractive power having a sign different from a sign of the refractive power of the image-motion correcting lens unit.

By making such an arrangement, it is possible to make the refractive power of the image-motion correcting lens unit large. As a result, it is possible to make an amount of shift in the imaging position large with respect to an amount of shift of the image-motion correcting lens unit. Accordingly, it is possible to carry out image-motion correction of higher accuracy with a small amount of shift.

Moreover, in the lens system according to the present embodiment, it is preferable that the image-motion correcting lens unit includes a plurality of lenses and a predetermined lens, and the plurality of lenses have a refractive power having a sign same as the sign of the refractive power of the image-motion correcting lens unit, and the predetermined lens has a refractive power having a sign different from the sign of the refractive power of the image-motion correcting lens unit.

Aberrations which occur when there is a shaking are mainly, the spherical aberration, a curvature of field, and a chromatic aberration of magnification. For reducing degradation of correction performance with respect to the shaking, it is necessary to reduce an amount of the aberrations which occur. Here, in the image-motion correcting lens unit, since a proportion of load of the refractive power being large (the refractive power being large), the aberration is susceptible to occur.

Therefore, the image-motion correcting lens unit is formed by the plurality of lenses and the predetermined lens. Moreover, by the plurality of lenses having the refractive power having a sign same as the sign of the refractive power of the image-motion correcting lens unit, it is possible to reduce occurrence of the spherical aberration and the curvature of field. Furthermore, by the predetermined lens having the refractive power having a sign different from the sign of the refractive power of the image-motion correcting lens unit, it is possible to correct the chromatic aberration favorably.

Furthermore, it is desirable to let the number of the plurality of lenses to be two and the number of the predetermined lenses to be one, and to form the image-motion correcting lens unit by a total of three lenses.

Moreover, in the lens system according to the present embodiment, it is preferable that the rear lens unit includes a first predetermined lens unit which is disposed immediately before the image-motion correcting lens unit, and a second predetermined lens unit which is disposed immediately after the image-motion correcting lens unit, and each of the first predetermined lens unit and the second predetermined lens unit has a refractive power having a sign different from the sign of the refractive power of the image-motion correcting lens unit.

By making such an arrangement, it is possible to make the refractive power of the image-motion correcting lens unit further larger, while suppressing the occurrence of aberration. As a result, it is possible to make the amount of shift in the imaging position further larger with respect to the amount of shift in the image-motion correcting lens unit. Accordingly, it is possible to carry out image-motion correction of higher accuracy with a small amount of shift.

Moreover, in the lens system according to the present embodiment, it is preferable that the image-motion correcting lens unit has a negative refractive power.

In the image-motion correction, the image-motion correcting lens unit is shifted. For the image-motion correction, it is preferable to make the amount of movement of the image-motion correcting lens unit small (to narrow a range of movement). For making the amount of movement small, it is desirable to let a lens unit (lens) having a smaller lens diameter to be the image-motion correcting lens unit. By letting the refractive power of the image-motion correcting lens unit to be a negative refractive power, it is possible to adopt an optical lay out in which, it is easy to make the lens diameter of the image-motion correcting lens unit small, and therefore it is preferable.

Moreover, in the lens system according to the present embodiment, it is preferable that the front lens unit includes in order from the object side to the image side, the first lens unit, the second lens unit, and the third lens unit.

For shortening the overall length of the lens system, it is necessary to arrange the lens system to include a telephoto arrangement, as well as to enhance an effect due to the telephoto arrangement. For enhancing the effect due to the telephoto arrangement, it is preferable to dispose a lens unit having a positive refractive power nearest to the object, and to dispose a lens unit having a negative refractive power on the image side thereof. By making such an arrangement, correction of the spherical aberration, the coma and the astigmatism becomes possible mainly, while enhancing the effect due to the telephoto arrangement.

Here, as aforementioned, the front lens unit includes in order from the object side to the image side, the lens unit having a positive refractive power and the lens unit having a negative refractive power. Therefore, even in the lens unit according to the present embodiment, since the effect due to the telephoto arrangement has been enhanced, it is possible to shorten the overall length of the lens system. Moreover, by making such an arrangement, it is possible to correct favorably mainly the spherical aberration, the coma, and the astigmatism, while enhancing the effect due to the telephoto arrangement.

More specifically, by letting the first lens unit to be the lens unit having a positive refractive power, and the second lens unit and the third lens unit to be the lens units having a negative refractive power, only three lens units are included in the front lens unit. By making such an arrangement, it is possible to shorten the overall length of the front lens unit.

Moreover, by including at least two lens units having a negative refractive power, correction of the spherical aberration, the coma, and the astigmatism in the front lens unit can be shared between the second lens unit and the third lens unit. Therefore, it is possible to correct favorably these aberrations in the front lens unit.

Moreover, by the second lens unit and the third lens unit having a negative refractive power, it is possible to make the negative refractive power of the front lens unit large without worsening these aberrations. Accordingly, since it is possible to enhance further the effect due to the telephoto arrangement, further shortening of the overall length of the optical system is possible.

Moreover, in the lens system according to the present embodiment, it is preferable that the front lens unit includes in order from the object side to the image side, the first lens unit, the second lens unit, and the third lens unit, and the rear lens unit includes a lens unit having a positive refractive power, and the lens unit having a positive refractive power is disposed nearest to the object, in the rear lens unit, and a position of the lens unit having a positive refractive power is fixed, and the lens unit having a positive refractive power includes the aperture stop.

The technical significance of including the first lens unit, the second lens unit, and the third lens unit in the front lens unit is as aforementioned.

On the image side of the aperture stop, it is possible to make the lens diameter small. Therefore, the aperture stop is disposed in a lens unit which is positioned nearest to be object, in the rear lens unit. By making such an arrangement, it is possible to make large the number of lens units in which, it is possible to make a lens diameter small. As a result of this, it is possible to make the optical system small-sized. Moreover, it is preferable that the refractive power of the lens unit which is disposed nearest to the object, which disposed in the rear lens unit, is a positive refractive power.

Moreover, by letting the lens unit having a positive refractive power to be fixed all the time, it is possible to reduce the number of lenses to be moved. Accordingly, it is possible to make light-weight the lens unit to be moved at the time of focusing.

Moreover, in the lens system according to the present embodiment, it is preferable that only the focusing lens unit is a lens unit which is movable in the optical axial direction.

By letting only the focusing lens unit to be a lens unit which is movable in the optical axial direction, it is possible to reduce the number of lenses which are to be moved. Accordingly, it is possible to make a lens unit which is to be moved at the time of focusing light-weight.

Moreover, in the lens system according to the present embodiment, it is preferable that only the focusing lens unit and the image-motion correcting lens unit are the lens units which are movable.

By letting only the focusing lens unit and the image-motion correcting lens unit to be the lens units which are movable, it is possible to reduce the number of lenses which are to be moved. Accordingly, it is possible to make the lens units which are to be moved at the time of focusing and at the time of image-motion correction light-weight.

Moreover, in the lens system according to the present embodiment, it is preferable that the focusing lens unit includes maximum of two lenses.

In the basic arrangement according to the present embodiment, as aforementioned, it is possible to correct the spherical aberration, the coma, and the astigmatism favorably in the front lens unit. Therefore, by disposing the focusing lens unit in the rear lens unit, it is possible to improve stability of focusing performance. Moreover, with the improvement in the stability of focusing performance, even if the focusing lens unit is formed by a small number of lenses, not more than two, securing improved focusing performance and making the focusing lens unit light-weight are possible.

Moreover, in the lens system according to the present embodiment, it is preferable that the focusing lens unit consists of two lenses, which are, one positive lens and one negative lens.

By forming the focusing lens unit by one positive lens and one negative lens, it is possible to reduce occurrence of the chromatic aberration in the focusing lens unit. As a result, it is possible to secure stable focusing performance at the time of focusing. Moreover, by carrying out the correction of chromatic aberration by the minimum number lenses which is two, it is possible to achieve both of securing the improved focusing performance and making the focusing lens unit light-weight.

Moreover, in the lens system according to the present embodiment, it is preferable that the rear lens unit includes a positive lens and a negative lens.

By disposing at least the positive lens and the negative lens in the rear lens unit, it is possible to suppress the occurrence of the spherical aberration and the coma in the rear lens unit. As a result, a favorable imaging performance is achieved. Moreover, the positive lens and the negative lens are to be disposed regardless of order of disposing.

Moreover, in the lens unit according to the present embodiment, it is preferable that the rear lens unit includes in order from the object side to the image side, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power, and the lens unit having a negative refractive power in the rear lens unit is an image-motion correcting lens unit, and the image-motion correcting lens unit moves in a direction different from a direction of the optical axis, to reduce an image motion due to shaking of the single focal length lens system.

The focusing lens unit is disposed in the front lens unit. It is preferable to let the refractive power of the focusing lens unit to be a negative refractive power, and to dispose a lens unit having a positive refractive power on the image side of the focusing lens unit. By making such an arrangement, since the magnification of the focusing lens unit is enhanced, it is possible to improve the focusing sensitivity more easily. Accordingly, since it is possible to reduce the amount of movement of the focusing lens unit at the time of focusing, it is possible to make the focusing speed high.

Moreover, in the image-motion correction, the image-motion correcting lens unit is shifted. For the image-motion correction, it is preferable to make the amount of movement of the image-motion correcting lens unit small (to narrow the range of movement). For making the amount of movement small, it is desirable to let a lens unit (lens) having a smaller lens diameter to be the image-motion correcting lens unit. By letting the refractive power of the image-motion correcting lens unit to be a negative refractive power, it is possible to adopt an optical lay out in which, it is easy to make the lens diameter of the image-motion correcting lens unit small, and therefore it is preferable.

Moreover, a lens unit having a positive refractive power is disposed on the object side of the image-motion correcting lens unit, and a lens unit having a positive refractive power is disposed on the image side of the image-motion correcting lens unit. By making such an arrangement, it is possible to make the refractive power of the image-motion correcting lens unit large. As a result, it is possible to make an amount of shift in the imaging position large with respect to an amount of shift of the image-motion correcting unit. Accordingly, it is possible to carry out image-motion correction of higher accuracy with a small amount of shift.

Meanwhile, the coma occurs due to shifting of the image-motion correcting lens unit. Therefore, if the focusing lens unit is disposed on the image side of the image-motion correcting lens unit, an effect of correction of the coma fluctuates substantially due to focusing. Therefore, it is not preferable to dispose the focusing lens unit on the image side of the image-motion correcting lens unit.

Moreover, the lens unit having a positive refractive power which is disposed on the object side of the image-motion correcting lens unit is also the positive lens unit disposed on the image side of the focusing lens unit. In such manner, when the lens unit disposed on the object side of the image-motion correcting lens unit and the lens unit disposed on the image side of the focusing lens unit are let to be common, it is possible to make an optical lay out of the rear lens unit simple.

Moreover, by disposing all the lens units in the rear lens unit on the image side of the aperture stop, it is possible to make the diameter of the rear lens unit even smaller.

Moreover, aberrations which occur when there is a shaking are mainly, the spherical aberration, the curvature of field, and the chromatic aberration of magnification. For reducing degradation of correction performance with respect to the shaking, it is necessary to reduce an amount of the aberrations which occur. Here, in the image-motion correcting lens unit, since the proportion of load of the refractive power being large (the refractive power being large), the aberration is susceptible to occur.

Therefore, the image-motion correcting lens unit is formed by the plurality of lenses and the predetermined lens. Moreover, by the plurality of lenses having the refractive power having a sign same as the sign of the refractive power of the image-motion correcting lens unit, it is possible to reduce occurrence of the spherical aberration and the curvature of field. Furthermore, by the predetermined lens having the refractive power having a sign different from the sign of the refractive power of the image-motion correcting lens unit, it is possible to correct the chromatic aberration favorably.

Moreover, it is desirable to let the plurality of lenses to be negative lenses, and the predetermined lens to be a positive lens, and to include at least one positive lens and two negative lenses.

Moreover, it is preferable that the focusing lens unit includes two lenses, and the lens unit having a positive refractive power, which is disposed between the focusing lens unit and the image-motion correcting lens unit, includes not more than two lenses, and the image-motion correcting lens unit includes three lenses. By making such an arrangement, it is possible to achieve a lens system having a small number of lenses, and in which, the focusing performance and correction performance with respect to the shaking are favorable.

Moreover, in the lens system according to the present embodiment, it is preferable that the two lens units having a positive refractive power in the rear lens unit are disposed such that, one lens unit having a positive refractive power is on the object side of the lens unit having a negative refractive power, immediately before the lens unit having a negative refractive power, and the other lens unit having a positive refractive power is on the image side of the lens unit having a negative refractive power, immediately after the lens unit having a negative refractive power, and both the lens units having a positive refractive power are lens units which do not move, and are fixed all the time.

As aforementioned, in a case in which, the rear lens unit includes two lens units having a positive refractive power and one lens unit having a negative refractive power, by making such an arrangement, it is possible to minimize the number of lens units that are to be moved. Therefore, making an image-motion correcting unit (an arrangement including the image-motion correcting lens unit and a moving mechanism) small-size and light-weight is facilitated.

Moreover, in the lens system according to the present embodiment, it is preferable to include an image-motion correcting lens unit which satisfies the following conditional expression (10): 0.8<|MG _(ISback)×(MG _(IS)−1)|<5.0  (10)

where,

MG_(IS) denotes a lateral magnification of the image-motion correcting lens unit in an arbitrary focused state, and

MG_(ISback) denotes a lateral magnification of an overall optical system between the image-motion correcting lens unit and the image plane, in an arbitrary focused state.

When falling below a lower limit value of conditional expression (10), an effect of image-motion correction by shifting the image-motion correcting lens unit is not achieved sufficiently. When exceeding an upper limit value of conditional expression (10), since a proportion of load of the refractive power on the image-motion correcting lens unit becomes large, degradation of the correction performance with respect to the shaking becomes large.

Moreover, in the lens system according to the present embodiment, it is preferable that the focusing lens unit satisfies the following conditional expression (11): 1.5<|(MG _(foback))²×{(MG _(fo))²−1}|<8.0  (11)

where,

MG_(fo) denotes a lateral magnification of the focusing lens unit in an arbitrary focused state, and

MG_(foback) denotes a lateral magnification of the overall optical system between the focusing lens unit and the image plane, in an arbitrary focused state.

When falling below a lower limit value of conditional expression (11), since the amount of movement of the focusing lens unit become excessively large, shortening the overall length of the lens system becomes difficult. When exceeding an upper limit value of conditional expression (11), since a position control of the focusing lens unit becomes difficult, it is not possible to carry out an accurate focusing.

Moreover, in the lens unit according to the present embodiment, it is preferable that the following conditional expression (12) is satisfied: 0.20<|f _(G1) /f _(G2)|<3.0  (12)

where,

f_(G1) denotes a focal length of the first lens unit, and

f_(G2) denotes the focal length of the second lens unit.

When falling below a lower limit value of conditional expression (12), it is advantageous for shortening the overall length of the lens system, but the refractive power of the first lens unit with a large aperture becomes excessively large. In this case, since a proportion of the refractive power of the first lens unit with respect to the refractive power of the overall lens system becomes large, it becomes difficult to make the overall lens system light-weight.

When exceeding an upper limit value of conditional expression (12), the refractive power of the second lens unit becomes excessively large. In this case, since a proportion of the refractive power of the second lens unit with respect to the refractive power of the overall lens system becomes large, shortening the overall length of the lens system becomes difficult.

Moreover, in the lens unit according to the present embodiment, it is preferable that the aperture stop is disposed on the image side of the second lens unit.

It is preferable to impart a positive refractive power to the overall front lens unit while enhancing the effect due to the telephoto arrangement in the front lens unit. Accordingly, since it is possible to reduce a proportion of load of refractive power on the lens unit positioned on the image side of the second lens unit, it is possible to shorten the overall length of the lens system, as well as to reduce an aberration which occurs in the rear lens unit. As a result of this, it is possible to secure a favorable imaging performance.

Here, by making the refractive power of the first lens unit and the second lens unit large, and by disposing the aperture stop on the image side of the second lens unit, it is possible to make small a lens diameter in the lens unit positioned on the image side of the aperture stop. Since the focusing lens unit is positioned on the image side of the aperture stop, making the focusing lens unit light-weight can be facilitated.

Moreover, the aperture stop can be deemed as one of the components that forms the rear lens unit, and can be deemed as a component that is independent of the front lens unit and the rear lens unit. In the latter case, the aperture stop is a component in a diaphragm unit. Therefore, by disposing the diaphragm unit on the image side of the second lens unit, it is possible to achieve the abovementioned effect.

Moreover, in the lens unit according to the present embodiment, it is preferable that the rear lens unit includes in order from the object side to the image side, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power, and the lens unit having a positive refractive power positioned on the object side of the lens unit having a negative refractive power in the rear lens unit includes two sub lens units having a positive refractive power, and the aperture stop is disposed to be sandwiched between the two sub lens units.

In the front lens unit, it is desirable to impart a positive refractive power to the overall front lens unit while enhancing an effect due to the telephoto arrangement. Accordingly, since it is possible to reduce a proportion of load of the refractive power in the rear lens unit, it is possible to shorten the overall length of the lens system as well as to reduce an aberration which occurs in the rear lens unit. As a result, it is possible to secure a favorable imaging performance.

Here, by making the positive refractive power of the front lens unit large, and by disposing the aperture stop immediately after the front lens unit, it is possible to make a diameter small in the lens unit positioned on the image side of the aperture stop.

By disposing the aperture stop to be sandwiched between the two sub lens unit, the lens unit having a positive refractive power is positioned on the object side of the aperture stop. Therefore, it is possible to make the diameter of the aperture stop small.

Moreover, it is preferable that the sub lens unit which is disposed on the object side of the aperture stop includes one lens. By making such an arrangement, it is possible to make the sub lens unit light-weight.

Moreover, the diaphragm unit may include the aperture stop and the sub lens unit which is disposed on the object side of the aperture stop. In this case, the diaphragm unit is disposed immediately after the front lens unit. Moreover, the diaphragm unit does not include a lens on the image side of the aperture stop, but include a lens having a positive refractive power on the object side of the aperture stop. Moreover, the lens having a positive refractive power is disposed adjacent to the aperture stop. By making such an arrangement, the abovementioned effect is achieved.

Moreover, in the lens system according to the present embodiment, it is preferable that the diaphragm unit does not include a lens.

By making such an arrangement, it is possible to reduce an effect of a manufacturing error (tilt and shift) between the front lens unit and the rear lens unit.

Moreover, in the lens system according to the present embodiment, it is preferable that the diaphragm unit does not include a lens, and is disposed nearest to the object side of the rear lens unit.

By making such an arrangement, it is possible to reduce an effect of a manufacturing error (tilt and shift) between the front lens unit and the rear lens unit.

Moreover, in the lens system according to the present embodiment, it is preferable that the diaphragm unit is disposed immediately after the front lens unit.

In the front lens unit, it is desirable to impart a positive refractive power to the overall front lens unit while enhancing an effect due to the telephoto arrangement. Accordingly, since it is possible to reduce a proportion of load of the refractive power in the rear lens unit, it is possible to shorten the overall length of the lens system as well as to reduce an aberration which occurs in the rear lens unit. As a result, it is possible to secure a favorable imaging performance.

Here, it is preferable that the diaphragm unit is disposed immediately after the front lens unit. By making such an arrangement, it is possible to make a diameter small in a lens unit on the image side of the diaphragm unit (aperture stop).

Moreover, in the lens system according to the present embodiment, it is preferable that the following conditional expression (13) is satisfied: 0<MG _(G2)  (13)

where,

MG_(G2) denotes a lateral magnification of the second lens unit at the time of focusing at an object at infinity.

When falling below a lower limit value of conditional expression (13), since the refractive power of the second lens unit becomes large, correction of the spherical aberration and the astigmatism becomes difficult. Moreover, since a light beam emerged from the second lens unit is diverged, a lens diameter in the rear lens unit becomes large. Therefore, small-sizing of a lens unit positioned on the image side of the second lens unit becomes difficult.

Moreover, in the lens system according to the present embodiment, it is preferable that the following conditional expression (14) is satisfied: 0≦|f/r _(G2b)|<7.0  (14)

where,

f denotes the focal length of the single focal length lens system at the time of focusing at an object at infinity, and

r_(G2b) denotes a paraxial radius of curvature of a lens surface on the object side of the focusing lens unit, immediately before the focusing lens unit.

When exceeding an upper limit value of conditional expression (14), an amount of occurrence of the spherical aberration and the coma increases at a lens surface immediately before the object side of the focusing lens unit. Since an effect of correction of these aberrations affect the focusing lens unit, it is not possible to secure a stable imaging performance at the time of focusing. Moreover, the lens surface on the object side of the focusing lens unit, immediately before the focusing lens unit, is a lens surface which is positioned on the object side of the focusing lens unit, and is a lens surface which is positioned nearest to the focusing lens unit.

Moreover, in the lens system according to the present embodiment, it is preferable that the following conditional expression (15) is satisfied: 0.5≦φ_(fo)/φ_(La)≦0.92  (15)

where,

φ_(fo) denotes a maximum effective aperture from among effective apertures of lenses in the focusing lens unit, and

φ_(La) denotes a maximum effective aperture of a lens positioned nearest to the image in the single focal length lens system.

When exceeding a lower limit value of conditional expression (15), it is possible to suppress the refractive power of the focusing lens unit from becoming large, and to reduce the number of lenses in the focusing lens unit. As a result, it is possible to make the focusing lens unit light-weight. When falling below an upper limit value of conditional expression (15), it is possible to suppress the refractive power of the focusing lens unit from becoming excessively small, and to make the diameter of the focusing lens unit small. Moreover, it is possible to make the amount of movement of the focusing lens unit at the time of focusing small. As a result, it is possible to make the focusing unit small, to shorten the overall length of the optical system, and also to make a diameter of a lens frame small.

Moreover, in a case in which, the focusing lens unit includes a plurality of lenses, φ_(fo) is a maximum effective aperture from among effective apertures of surfaces of lenses. Moreover, a lens positioned nearest to the image has an object-side surface and an image-side surface. Therefore, φ_(La) is a maximum effective aperture from among an effective aperture of the object-side surface and an effective aperture of the image-side surface.

Moreover, in the lens system according to the present embodiment, it is preferable that the following conditional expression (16) is satisfied: 0.023≦D _(sfo) /D _(LTL)≦0.110  (16)

where,

D_(sfo) denotes a distance on the optical axis from the aperture stop up to a lens surface nearest to an object of the focusing lens unit,

D_(LTL) denotes a distance on the optical axis from a lens surface nearest to the object of the single focal length lens system up to an image plane, and

both D_(sfo) and D_(LTL) are distances at the time of focusing at an object at infinity.

In the lens system according to the present embodiment, a light beam is converged by using a positive refractive power of a lens unit which is positioned before the aperture stop. When exceeding a lower limit value of conditional expression (16), it is possible to achieve sufficiently an effect of converging the light beam. Therefore, it is possible to suppress the diameter of the focusing lens unit from becoming large. When falling below an upper limit value of conditional expression (16), it is possible to shorten the overall length of the optical system.

Moreover, in the lens system according to the present embodiment, it is preferable that the following conditional expression (17) is satisfied: 0.2≦D _(sfo)/φ_(s)≦0.8  (17)

where,

D_(sfo) denotes the distance on the optical axis from the aperture stop up to a lens surface nearest to an object of the focusing lens unit, and is a distance at the time of focusing at an object at infinity, and

Φ_(s) denotes a maximum diameter of the aperture stop.

In the lens system according to the present embodiment, a light beam is converged by using a positive refractive power of a lens unit which is positioned before the aperture stop. When exceeding a lower limit value of conditional expression (17), it is possible to achieve sufficiently an effect of converging the light beam. Therefore, it is possible to make the diameter of the focusing lens unit small. When falling below an upper limit value of conditional expression (17), it is possible to shorten the overall length of the optical system.

Moreover, in the lens system according to the present embodiment, it is preferable that an optical system positioned on the image side of the focusing lens unit includes at least two positive lenses and one negative lens.

If the small-sizing of the focusing lens unit is carried out, the refractive power of the focusing lens unit becomes large. Therefore, in the focusing lens unit, the amount of occurrence of the spherical aberration, the longitudinal chromatic aberration, and the astigmatism tends to increase mainly. Here, the optical system positioned on the image side of the focusing lens unit has a positive refractive power. For suppressing a fluctuation in these aberrations at the time of focusing, it is preferable to make the amount of occurrence of these aberrations small in the optical system on the image side of the focusing lens unit.

The optical system positioned on the image side of the focusing lens unit is formed by one positive lens and one negative lens. At this time, by making Abbe number for the negative lens to be smaller than Abbe number for the positive lens, it is possible to suppress the occurrence of the chromatic aberration and the spherical aberration. Moreover, by using one more positive lens, it is possible to suppress the occurrence of the astigmatism easily. Moreover, for making the occurrence of these aberrations even smaller, it is preferable that the optical system positioned on the image side of the focusing lens unit includes at least two positive lenses.

Moreover, an image pickup apparatus according to the present embodiment includes an optical system, and an image pickup element which has an image pickup surface, and which converts an image formed on the image pickup surface by the optical system to an electric signal, and the optical system is one of the single focal length lens systems according to the aforementioned embodiments.

It is possible to realize an image pickup apparatus which has a superior mobility, and which is capable of carrying out photography with high resolution.

It is preferable that a plurality of abovementioned arrangements is satisfied simultaneously.

Moreover, for each conditional expression, it is preferable to restrict both or one of the upper limit value and the lower limit value, as it enables to have that function more assuredly.

For conditional expression (1), it is more preferable to let the lower limit value to be 0.1.

Moreover, for conditional expression (1), it is more preferable to let the upper limit value to be 0.3.

For conditional expression (2), it is more preferable to let the lower limit value to be 0.05.

Moreover, for conditional expression (2), it is more preferable to let the upper limit value to be 0.18.

For conditional expression (3), it is more preferable to let the lower limit value to be 6, and 11 is even more preferable.

Moreover, for conditional expression (3), it is more preferable to let the upper limit value to be 30, and 27 is even more preferable.

For conditional expression (4), it is more preferable to let the lower limit value to be 38.

Moreover, for conditional expression (4), it is more preferable to let the upper limit value to be 45.

For conditional expression (5), it is more preferable to let the lower limit value to be 0.62.

Moreover, for conditional expression (5), it is more preferable to let the upper limit value to be 0.8.

For conditional expression (6), it is more preferable to let the lower limit value to be 2.0.

Moreover, for conditional expression (6), it is more preferable to let the upper limit value to be 4.5.

For conditional expression (7), it is more preferable to let the lower limit value to be −0.3.

Moreover, for conditional expression (7), it is more preferable to let the upper limit value to be −0.12.

For conditional expression (8), it is more preferable to let the lower limit value to be 0.4.

Moreover, for conditional expression (8), it is more preferable to let the upper limit value to be 1.1.

For conditional expression (9), it is more preferable to let the lower limit value to be 0.12.

Moreover, for conditional expression (9), it is more preferable to let the upper limit value to be 0.47.

For conditional expression (10), it is more preferable to let the lower limit value to be 1.3.

Moreover, for conditional expression (10), it is more preferable to let the upper limit value to be 3.5.

For conditional expression (11), it is more preferable to let the lower limit value to be 2.5.

Moreover, for conditional expression (11), it is more preferable to let the upper limit value to be 6.5.

For conditional expression (12), it is more preferable to let the lower limit value to be 0.23.

Moreover, for conditional expression (12), it is more preferable to let the upper limit value to be 1.0.

For conditional expression (14), it is more preferable to let the upper limit value to be 6.5. It is even more preferable to let the upper limit value to be 4.0, and 2.0 is all the more preferable.

For conditional expression (15), it is more preferable to let the lower limit value to be 0.6.

Moreover, for conditional expression (15), it is more preferable to let the upper limit value to e 0.88, and 0.85 is even more preferable.

For conditional expression (16), it is more preferable to let the lower limit value to be 0.025, and 0.04 is even more preferable.

Moreover, for conditional expression (16), it is more preferable to let the upper limit value to be 0.1, and 0.090 is even more preferable.

For conditional expression (17), it is more preferable to let the lower limit value to be 0.3, and 0.45 is even more preferable.

Moreover, for conditional expression (17), it is more preferable to let the upper limit value to be 0.75, and 0.7 is even more preferable.

Moreover, the aforementioned single focal length lens system and the image pickup apparatus may satisfy the plurality of arrangements simultaneously. Making such an arrangement is preferable for achieving a favorable single focal length lens system and an image pickup apparatus. Moreover, combinations of preferable arrangements are arbitrary. Moreover, for each conditional expression, only an upper limit value or a lower limit value of a numerical range of a conditional expression further restricted may be restricted.

Examples of the single focal length lens system according to the present invention will be described below in detail by referring to the accompanying diagrams. However, the present invention is not restricted to the examples described below.

Examples from an example 1 to an example 4 of a single focal length lens system according to the present invention will be described below. Lens cross-sectional views at the time of focusing at an object at infinity of the examples from the example 1 to the example 4 are shown in FIG. 1A, FIG. 1B, FIG. 2A, and FIG. 2B.

In these lens cross-sectional views, a first lens unit is denoted by G1, a second lens unit is denoted by G2, a third lens unit is denoted by G3, a fourth lens unit is denoted by G4, a fifth lens unit is denoted by G5, a sixth lens unit is denoted by G6, and an image plane is denoted by I. Moreover, although it is not shown in the diagrams, a parallel flat plate which forms a low-pass filter, and a cover glass of an electronic image pickup element, may be disposed between the last lens unit and the image plane. A wavelength region restricting coating which restricts infrared light may be applied on a surface of the parallel flat plate. Moreover, a multilayer film for restricting a wavelength region may be formed on a surface of the cover glass. Furthermore, an arrangement may be made to impart a low-pass filter effect to the cover glass. Moreover, an image pickup element may be disposed on the image plane I.

In each example, a single focal length lens system includes in order from an object side to an image side, a front lens unit GF and a rear lens unit GR. Here, in the example 3 and the example 4, the rear lens unit GR includes an aperture stop S.

A single focal length lens system according to the example 1, as shown in FIG. 1A, includes in order from an object side to an image side, a front lens unit GF having a positive refractive power and a rear lens unit GR having a positive refractive power. The front lens unit GF includes an aperture stop S. Here, r16 is the aperture stop and r13 and r24 are virtual surfaces.

The front lens unit GF include in order from the object side to the image side, a first lens unit G1 having a positive refractive power, a second lens unit G2 having a negative refractive power, and a third lens unit G3 having a negative refractive power. The rear lens unit GR includes a fourth lens unit G4 having a positive refractive power, a fifth lens unit G5 having a negative refractive power, and a sixth lens unit G6 having a positive refractive power.

The first lens unit G1 includes a biconvex positive lens L1, a biconvex positive lens L2, a biconcave negative lens L3, a negative meniscus lens L4 having a convex surface directed toward the object side, and a positive meniscus lens L5 having a convex surface directed toward the object side. Here, the negative meniscus lens L4 and the positive meniscus lens L5 are cemented.

Moreover, a 1-1^(th) sub lens unit includes the biconvex positive lens L1, the biconvex positive lens L2, and the biconcave negative lens L3. A 1-2^(th) sub lens unit include the negative meniscus lens L4 and the positive meniscus lens L5.

The second lens unit G2 includes a biconvex positive lens L6, a biconcave negative lens L7, and a biconcave negative lens L8. Here, the biconvex positive lens L6 and the biconcave negative lens L7 are cemented.

The third lens unit G3 includes a biconvex positive lens L9 and a biconcave negative lens L10. The third lens unit G3 is a focusing lens unit, and moves toward the image side along an optical axis at the time of focusing from an object at infinity to an object at a close distance.

The fourth lens unit G4 includes a negative meniscus lens L11 having a convex surface directed toward the object side and a biconvex positive lens L12. Here, the negative meniscus lens L11 and the biconvex positive lens L12 are cemented.

The fifth lens unit G5 includes a biconvex positive lens L13, a biconcave negative lens L14, and a biconcave negative lens L15. The fifth lens unit G5 is an image-motion correcting lens unit, and moves in a direction different from an optical axial direction, such as a direction orthogonal to the optical axis, at the time of correcting image motion.

The sixth lens unit G6 includes a biconvex positive lens L16 and a biconvex positive lens L17.

A single focal length lens system according to the example 2, as shown in FIG. 1B, includes in order from an object side to an image side, a front lens unit GF having a positive refractive power and a rear lens unit GR having a positive refractive power. The front lens unit GF includes an aperture stop S. Here, r15 is the aperture stop, and r23 is a virtual surface.

The front lens unit GF includes in order from the object side to the image side, a first lens unit G1 having a positive refractive power, a second lens unit G2 having a negative refractive power, and a third lens unit G3 having a negative refractive power. The rear lens unit GR includes a fourth lens unit G4 having a positive refractive power, a fifth lens unit G5 having a negative refractive power, and a sixth lens unit G6 having a positive refractive power.

The first lens unit G1 includes a positive meniscus lens L1 having a convex surface directed toward the object side, a biconvex positive lens L2, a negative meniscus lens L3 having a convex surface directed toward the object side, a negative meniscus lens L4 having a convex surface directed toward the object side, and a biconvex positive lens L5. Here, the negative meniscus lens L4 and the biconvex positive lens L5 are cemented.

Moreover, a 1-1^(th) sub lens unit includes the positive meniscus lens L1, the biconvex positive lens L2, and the negative meniscus lens L3. A 1-2^(th) sub lens unit includes the negative meniscus lens L4 and the biconvex positive lens L5.

The second lens unit G2 includes a biconvex positive lens L6, a biconcave negative lens L7, and a biconcave negative lens L8. Here, the biconvex positive lens L6 and the biconcave negative lens L7 are cemented.

The third lens unit G3 includes a biconvex positive lens L9 and a biconcave negative lens L10. The third lens unit G3 is a focusing lens unit, and moves toward the image side along an optical axis at the time of focusing from an object at infinity to an object at a close distance.

The fourth lens unit G4 includes a negative meniscus lens L11 having a convex surface directed toward the object side and a biconvex positive lens L12. Here, the negative meniscus lens L11 and the biconvex positive lens L12 are cemented.

The fifth lens unit G5 includes a biconvex positive lens L13, a biconcave negative lens L14, and a biconcave negative lens L15. The fifth lens unit G5 is an image-motion correcting lens unit, and moves in a direction different from an optical axial direction, such as a direction orthogonal to the optical axis, at the time of correcting image motion.

The sixth lens unit G6 includes a biconvex positive lens L16 and a biconvex positive lens L17.

A single focal length lens system according to the example 3, as shown in FIG. 2A, includes in order from an object side to an image side, a front lens unit GF having a positive refractive power and a rear lens unit GR having a positive refractive power. The rear lens unit GR includes an aperture stop S. Here, r15 is the aperture stop, and there is no virtual surface.

The front lens unit GF includes in order from the object side to the image side, a first lens unit G1 having a positive refractive power, a second lens unit G2 having a negative refractive power, and a third lens unit G3 having a negative refractive power. The rear lens unit GR includes a fourth lens unit G4 having a positive refractive power, a fifth lens unit G5 having a negative refractive power, and a sixth lens unit G6 having a positive refractive power.

The first lens unit G1 includes a biconvex positive lens L1, a positive meniscus lens L2 having a convex surface directed toward the object side, a negative meniscus lens L3 having a convex surface directed toward the object side, a negative meniscus lens L4 having a convex surface directed toward the object side, and a biconvex positive lens L5. Here, the positive meniscus lens L2 and the negative meniscus lens L3 are cemented. Moreover, the negative meniscus lens L4 and the biconvex positive lens L5 are cemented.

Moreover, a 1-1^(th) sub lens unit includes the biconvex positive lens L1, the positive meniscus lens L2, and the negative meniscus lens L3. A 1-2^(th) sub lens unit includes the negative meniscus lens L4 and the biconvex positive lens L5.

The second lens unit G2 includes a biconvex positive lens L6 and a biconcave negative lens L7. Here, the biconvex positive lens L6 and the biconcave negative lens L7 are cemented.

The third lens unit G3 includes a biconvex positive lens L8 and a biconcave negative lens L9. Here, the biconvex positive lens L8 and the biconcave negative lens L9 are cemented. The third lens unit G3 is a focusing lens unit, and moves toward the image side along an optical axis at the time of focusing from an object at infinity to an object at a close distance.

The fourth lens unit G4 includes a negative meniscus lens L10 having a convex surface directed toward the object side and a biconvex positive lens L11. Here, the negative meniscus lens L10 and the biconvex positive lens L11 are cemented.

The fifth lens unit G5 includes a biconvex positive lens L12, a biconcave negative lens L13, and a biconcave negative lens L14. Here, the biconvex positive lens L12 and the biconcave negative lens L13 are cemented. The fifth lens unit G5 is an image-motion correcting lens unit, and moves in a direction different from an optical axial direction, such as a direction orthogonal to the optical axis, at the time of correcting image motion.

Sixth lens unit G6 includes a biconvex positive lens L15, a negative meniscus lens L16 having a convex surface directed toward the object side, and a positive meniscus lens L17 having a convex surface directed toward the object side. The negative meniscus lens L16 and the positive meniscus lens L17 are cemented.

A single focal length lens system according to the example 4 of the present invention, as shown in FIG. 2B, includes in order from an object side to an image side, a front lens unit GF having a positive refractive power and a rear lens unit GR having a positive refractive power. The rear lens unit GR includes an aperture stop S. Here, r18 is the aperture stop, and r9 and r29 are virtual surfaces.

The front lens unit GF includes in order from the object side to the image side, a first lens unit G1 having a positive refractive power, a second lens unit G2 having a negative refractive power, and a third lens unit G3 having a negative refractive power. The rear lens unit GR includes a fourth lens unit G4 having a positive refractive power, a fifth lens unit G5 having a negative refractive power, and sixth lens unit G6 having a positive refractive power.

The first lens unit G1 includes a positive meniscus lens L1 having a convex surface directed toward the object side, a biconvex positive lens L2, a biconcave negative lens L3, a negative meniscus lens L4 having a convex surface directed toward the object side, and a positive meniscus lens L5 having a convex surface directed toward the object side. Here, the biconvex positive lens L2 and the biconcave negative lens L3 are cemented. Moreover, the negative meniscus lens L4 and the positive meniscus lens L5 are cemented.

Moreover, a 1-1^(th) sub lens unit includes the positive meniscus lens L1, the biconvex positive lens L2, and the biconcave negative lens L3. A 1-2^(th) sub lens unit includes the negative meniscus lens L4 and the positive meniscus lens L5.

The second lens unit G2 includes a biconvex positive lens L6 and a biconcave negative lens L7. Here, the biconvex positive lens L6 and the biconcave negative lens L7 are cemented.

The third lens unit G3 includes a biconvex positive lens L8 and a biconcave negative lens L9. Here, the biconvex positive lens L8 and the biconcave negative lens L9 are cemented. The third lens unit G3 is a focusing lens unit, and moves toward the image side along an optical axis at the time of focusing from an object at infinity to an object at a close distance.

The fourth lens unit G4 includes a positive meniscus lens L10 having a convex surface directed toward the object side, a negative meniscus lens L11 having a convex surface directed toward the object side, and a biconvex positive lens L12. Here, the negative meniscus lens L11 and the biconvex positive lens L12 are cemented. The aperture stop S is disposed between the positive meniscus lens L10 and the negative meniscus lens L11.

The fifth lens unit G5 includes a biconvex positive lens L13, a biconcave negative lens L14, and a biconcave negative lens L15. The biconvex positive lens L13 and the biconcave negative lens L14 are cemented. The fifth lens unit G5 is an image-motion correcting lens unit, and moves in a direction different from an optical axial direction, such as a direction orthogonal to the optical axis, at the time of correcting image motion.

The sixth lens unit G6 includes a biconvex positive lens L16 and a positive meniscus lens L17 having a convex surface directed toward the object side.

Numerical data of each example is shown below. Apart from the aforementioned symbols, r denotes a radius of curvature of a lens surface, d denotes a distance between lens surfaces, nd denotes a refractive index for a d-line of each lens, and νd denotes Abbe number for each lens. Moreover f denotes a focal length of the overall single focal length lens system, FNO. denotes an F-number, co denotes a half angle of view, IH denotes an image height, FB denotes a back focus, and Lens total length is a distance from a lens surface nearest to the object of the single focal length lens system up to a lens surface nearest to the image of the single focal length lens system. FB (back focus) is a value which is a distance from the last lens surface up to a paraxial image plane expressed upon air conversion. Moreover, the unit of length for each numerical value is mm, and the unit of angle is ° (degree).

Moreover, Infinity indicates the time of focusing at an object at infinity and Close distance indicates the time of focusing at an object at a close distance. Here, values in a column of close distance are values in a state of being focused at an object at a closest distance. A practical distance between an object and an image in the state of being focused at an object at a closest distance is 1.4 m in the examples 1, 2, 3, and 4.

EXAMPLE 1

Unit mm Surface data Surface no. r d nd νd  1 259.889 6.34 1.48749 70.23  2 −573.469 23.00  3 59.198 12.89 1.49700 81.54  4 −598.390 0.10  5 −1538.065 2.00 1.83481 42.71  6 159.680 40.00  7 47.697 2.00 1.79952 42.22  8 30.970 8.18 1.43875 94.93  9 4914.671 1.60 10 130.449 5.39 1.75520 27.51 11 −61.733 2.00 1.91082 35.25 12 149.010 2.36 13 ∞ 1.81 14 −377.736 1.50 1.79952 42.22 15 575.945 8.25 16(Stop) ∞ Variable 17 368.821 2.20 1.84666 23.78 18 −118.108 0.10 19 −109.311 0.90 1.80139 45.45 20 41.551 Variable 21 39.876 1.00 1.92286 18.90 22 23.568 5.08 1.61272 58.72 23 −175.976 0.10 24 ∞ 3.00 25 152.120 2.69 1.84666 23.78 26 −65.350 0.10 27 −240.179 0.90 1.80400 46.57 28 29.838 5.04 29 −26.216 0.80 1.61800 63.33 30 44.073 3.30 31 72.691 3.85 1.72342 37.95 32 −66.348 6.31 33 52.000 5.50 1.52249 59.84 34 −90.040 Image plane ∞ Various data Infinity Close distance f 292.32 FNO. 4.01 2ω(Angle of view) 4.37 IH 11.15 FB 35.98 Lens total length 228.46 228.46 d16 6.00 28.18 d20 28.18 6.00 Surface data Surface no. r d nd νd  1 132.985 6.746 1.48749 70.23  2 833.805 6.500  3 56.824 13.500 1.49700 81.54  4 −18692.587 0.100  5 2292.831 2.000 1.83481 42.71  6 194.055 22.000  7 58.436 2.000 1.79952 42.22  8 31.339 11.474 1.43875 94.93  9 −283.074 1.600 10 114.275 6.289 1.75520 27.51 11 −67.671 2.000 1.91082 35.25 12 130.501 2.322 13 −202.441 1.500 1.78590 44.20 14 1233.704 16.000 15(Stop) ∞ Variable 16 282.863 2.200 1.84666 23.78 17 −124.870 0.100 18 −110.471 0.900 1.80139 45.45 19 38.312 Variable 20 39.801 1.000 1.92286 18.90 21 22.065 4.963 1.58267 46.42 22 −136.632 0.100 23 ∞ 3.000 24 112.013 3.000 1.84666 23.78 25 −47.684 0.100 26 −51.871 0.900 1.80400 46.57 27 24.988 5.044 28 −37.965 0.800 1.69680 55.53 29 77.357 3.300 30 95.000 3.393 1.72342 37.95 31 −82.082 0.100 32 55.512 5.500 1.51633 64.14 33 −51.010 Image plane ∞ Various data Infinity Close distance f 293.57 FNO. 4.06 2ω(Angle of view) 4.32 IH 11.15 FB 39.18 Lens total length 198.68 d15 6.00 24.85 d19 25.07 6.21

EXAMPLE 3

Unit mm Surface data Surface no. r d nd νd  1 265.067 5.20 1.58913 61.14  2 −1490.468 1.00  3 59.582 12.50 1.43875 94.93  4 435.993 2.00 1.69680 55.53  5 172.557 26.62  6 57.861 2.00 1.77250 49.60  7 33.305 13.30 1.43875 94.93  8 −2433.738 3.83  9 1376.956 5.40 1.62588 35.70 10 −79.834 1.50 1.79952 42.22 11 265.360 Variable 12 405.447 2.00 1.80810 22.76 13 −4629.782 1.00 1.64000 60.08 14 51.816 Variable 15(Stop) ∞ 2.04 16 43.323 1.76 1.84666 23.78 17 28.126 4.81 1.48749 70.23 18 −260.562 12.64 19 76.376 3.00 1.80518 25.42 20 −77.000 1.00 1.60300 65.44 21 24.084 2.89 22 −57.663 1.20 1.83481 42.71 23 64.994 3.50 24 45.024 3.80 1.65100 56.16 25 −77.402 2.19 26 40.470 1.50 1.75500 52.32 27 19.851 4.50 1.56732 42.82 28 80.023 Image plane ∞ Various data Infinity Close distance f 294.33 FNO. 4.08 2ω(Angle of view) 4.31 IH 11.15 FB 45.04 Lens total length 213.64 213.64 d11 17.02 41.92 d14 30.40 5.50

EXAMPLE 4

Unit mm Surface data Surface no. r d nd νd  1 194.243 5.20 1.48749 70.23  2 1167.575 1.00  3 70.800 12.40 1.43875 94.93  4 −1405.411 2.00 1.69100 54.82  5 388.125 28.63  6 70.749 2.00 1.78800 47.37  7 39.267 11.00 1.43875 94.93  8 412.816 0.00  9 ∞ 5.01 10 115.990 6.10 1.60342 38.03 11 −153.511 1.50 1.83481 42.71 12 146.105 Variable 13 807.147 1.80 1.78472 25.68 14 −307.862 1.00 1.64000 60.08 15 53.770 Variable 16 72.543 2.80 1.84666 23.78 17 150.000 2.40 18(Stop) ∞ 1.80 19 66.610 1.00 1.84666 23.78 20 28.500 4.24 1.51633 64.14 21 −237.459 15.51 22 126.318 3.00 1.80518 25.42 23 −65.633 1.00 1.60300 65.44 24 24.000 3.25 25 −44.111 1.20 1.79952 42.22 26 197.168 3.50 27 57.692 3.90 1.65100 56.16 28 −56.296 10.54 29 ∞ 2.00 30 140.002 3.00 1.67270 32.10 31 600.028 Image plane ∞ Various data Infinity Close distance f 291.95 FNO. 4.05 2ω(Angle of view) 4.36 IH 11.15 FB 40.77 Lens total length 228.55 228.55 d12 21.14 47.50 d15 29.86 3.50

Aberration diagrams of examples from the example 1 to the example 4 described above are shown in FIG. 3A, FIG. 3B, FIG. 3C, FIG. 3D, FIG. 3E, FIG. 3F, FIG. 3G, and FIG. 3H to FIG. 6A, FIG. 6B, FIG. 6C, FIG. 6D, FIG. 6E, FIG. 6F, FIG. 6G, and FIG. 6H respectively. In each aberration diagram, FIY denotes the maximum image height.

In these aberration diagrams, FIG. 3A, FIG. 4A, FIG. 5A, and FIG. 6A show a spherical aberration (SA) at the time of focusing at an object at infinity, FIG. 3B, FIG. 4B, FIG. 5B, and FIG. 6B show an astigmatism (AS) at the time of focusing at an object at infinity, FIG. 3C, FIG. 4C, FIG. 5C, and FIG. 6C show a distortion (DT) at the time of focusing at an object at infinity, and FIG. 3D, FIG. 4D, FIG. 5D, and FIG. 6D show a chromatic aberration of magnification (CC) at the time of focusing at an object at infinity.

Moreover, FIG. 3E, FIG. 4E, FIG. 5E, and FIG. 6E show a spherical aberration (SA) at the time of focusing at an object at a close distance, FIG. 3F, FIG. 4F, FIG. 5F, and FIG. 6F show a astigmatism (AS) at the time of focusing at an object at a close distance, FIG. 3G, FIG. 4G, FIG. 5G, and FIG. 6G show a distortion (DT) at the time of focusing at an object at a close distance, and FIG. 3H, FIG. 4H, FIG. 5H, and FIG. 6H show a chromatic aberration of magnification (CC) at the time of focusing at an object at a close distance.

Next, values of conditional expressions (1) to (17) in each example are given below. Moreover, ‘-’ (hyphen) indicates that there is no corresponding arrangement or conditional expression is not satisfied.

Exam- Exam- Exam- Exam- Conditional expressions ple1 ple2 ple3 ple4 (1) |f_(G3)/f| 0.20 0.19 0.33 0.33 (2) f_(b)/f 0.12 0.13 0.15 0.14 (3) ν_(G2nMAX) − ν_(G2pMIN) 14.71 16.69 6.52 4.68 (4) ν_(G2nMAX) 42.22 44.2 42.22 42.71 (5) L_(TL)/f 0.78 0.68 0.73 0.78 (6) f_(G2)/f_(G3) 2.78 2.67 2.17 5.62 (7) f_(fo)/f −0.20 −0.19 −0.33 −0.33 (8) D_(G1)/f_(G1) 0.90 0.69 0.59 0.47 (9) D_(G112)/f_(G1) 0.38 0.24 0.25 0.22 (10) |MG_(ISback) × (MG_(IS) − 1)| 1.99 2 2.01 1.98 (11) |(MG_(foback))² − 3.85 4.49 3.57 3.27 {(MG_(fo))² − 1}| (12) |f_(G1)/f_(G2) 0.65 0.64 0.50 0.25 (13) MG_(G2) 1.33 1.38 1.36 1.13 (14) |f/r_(G2b)| 1.06 0.24 1.11 2.00 (15) Φ_(fo)/Φ_(La) 0.81 0.82 1.40 1.28 (16) D_(sfo)/D_(LTL) 0.026 0.030 — — (17) D_(sfo)/φ_(s) 0.25 0.25 — —

FIG. 7 is a cross-sectional view of a single-lens mirrorless camera as an electronic image pickup apparatus. In FIG. 7, a taking lens system 2 is disposed inside a lens barrel of a single-lens mirrorless camera 1. Amount portion 3 enables the taking lens system 2 to be detachable from a body of the single-lens mirrorless camera 1. As the mount portion 3, a mount such as a screw-type mount and a bayonet-type mount is to be used. In this example, a bayonet-type mount is used. Moreover, an image pickup element surface 4 and a back monitor 5 are disposed in the body of the single-lens mirrorless camera 1. As an image pickup element, an element such as a small-size CCD (charge coupled device) or a CMOS (complementary metal-oxide semiconductor) is to be used.

Moreover, as the taking lens system 2 of the single-lens mirrorless camera 1, the single focal length lens system according to the present invention described in any one of the examples from the first example to the fourth example is to be used.

FIG. 8 and FIG. 9 are conceptual diagrams of an arrangement of the image pickup apparatus according to the present invention. FIG. 8 is a front perspective view showing an appearance of a digital camera 40 as the image pickup apparatus, and FIG. 9 is a rear perspective view of the digital camera 40. The single focal length lens system according to the present invention is used in a photographic optical system 41 of the digital camera 40.

The digital camera 40 according to the present embodiment includes the photographic optical system 41 which is positioned in a photographic optical path 42, a shutter button 45, and a liquid-crystal display monitor 47. As the shutter button 45 disposed on an upper portion of the digital camera 40 is pressed, in conjunction with the pressing of the shutter button 45, photography is carried out by the photographic optical system 41 such as the single focal length lens system according to the first example. An object image which is formed by the photographic optical system 41 is formed on an image pickup element (photoelectric conversion surface) which is provided near an image forming surface. The object image which has been received optically by the image pickup element is displayed on the liquid-crystal display monitor 47 which is provided to a rear surface of the camera, as an electronic image by a processing means. Moreover, it is possible to record the electronic image which has been photographed, in a recording means.

FIG. 10 is a structural block diagram of an internal circuit of main components of the digital camera 40. In the following description, the processing means described above includes for instance, a CDS/ADC section 24, a temporary storage memory 117, and an image processing section 18, and a storage means consists of a storage medium section 19 for example.

As shown in FIG. 10, the digital camera 40 includes an operating section 12, a control section 13 which is connected to the operating section 12, the temporary storage memory 17 and an imaging drive circuit 16 which are connected to a control-signal output port of the control section 13, via a bus 14 and a bus 15, the image processing section 18, the storage medium section 19, a display section 20, and a set-information storage memory section 21.

The temporary storage memory 17, the image processing section 18, the storage medium section 19, the display section 20, and the set-information storage memory section 21 are structured to be capable of mutually inputting and outputting data via a bus 22. Moreover, the CCD 49 and the CDS/ADC section 24 are connected to the imaging drive circuit 16.

The operating section 12 includes various input buttons and switches, and informs the control section 13 of event information which is input from outside (by a user of the digital camera) via these input buttons and switches. The control section 13 is a central processing unit (CPU), and has a built-in computer program memory which is not shown in the diagram. The control section 13 controls the entire digital camera 40 according to a computer program stored in this computer program memory.

The CCD 49 is driven and controlled by the imaging drive circuit 16, and which converts an amount of light for each pixel of the object image to an electric signal, and outputs to the CDS/ADC section 24.

The CDS/ADC section 24 is a circuit which amplifies the electric signal which is input from the CCD 49, and carries out analog/digital conversion, and outputs to the temporary storage memory 17 image raw data (Bayer data, hereinafter called as ‘RAW data’) which is only amplified and converted to digital data.

The temporary storage memory 17 is a buffer which includes an SDRAM (Synchronous Dynamic Random Access Memory) for example, and is a memory device which stores temporarily the RAW data which is output from the CDS/ADC section 24. The image processing section 18 is a circuit which reads the RAW data stored in the temporary storage memory 17, or the RAW data stored in the storage medium section 19, and carries out electrically various image-processing including the distortion correction, based on image-quality parameters specified by the control section 13.

The storage medium section 19 is a recording medium in the form of a card or a stick including a flash memory for instance, detachably mounted. The storage medium section 19 records and maintains the RAW data transferred from the temporary storage memory 17 and image data subjected to image processing in the image processing section 18 in the card flash memory and the stick flash memory.

The display section 20 includes the liquid-crystal display monitor, and displays images and operation menu on the liquid-crystal display monitor. The set-information storage memory section 21 includes a ROM section in which various image quality parameters are stored in advance, and a RAM section which stores image quality parameters which are selected by an input operation on the operating section 12, from among the image quality parameters which are read from the ROM section.

In the digital camera 40 in which such an arrangement is made, by adopting the single focal length lens system according to the present invention as the photographing optical system 41, since it is possible to make the overall optical system light-weight and to make a focusing speed high, while maintaining a superior focusing performance, the digital camera 40 has a superior mobility, and enables to carry out photography with high resolution. Moreover, it is possible to use the single focal length lens system according to the present invention in an image pickup apparatus of a type having a quick-return mirror.

According to the present invention, it is possible to provide a single focal length lens system which has a superior mobility, and in which, aberrations are corrected favorably, and an image pickup apparatus using the single focal length lens system.

As described heretofore, in the single focal length lens system according to the present invention, since it is possible to shorten the overall length of the optical system and to make the focusing lens unit light-weight, it is possible to realize making the optical system light-weight. Moreover, by making the focusing lens unit light-weight, it is possible to make the focusing unit small-sized and light-weight, and accordingly it becomes easy to make the focusing speed high. For these reasons, the single focal length lens system according to the present invention is suitable for a single focal length lens system which has a superior mobility, and in which, aberrations are corrected favorably. Particularly, the single focal length lens system according to the present invention is useful for a telephoto lens and an ultra-telephoto lens. Moreover, the image pickup apparatus according to the present invention is suitable for an image pickup apparatus which has a superior mobility, and which is capable of carrying out photography with high resolution. 

What is claimed is:
 1. A single focal length lens system comprising in order from an object side to an image side along an optical axis: a front lens unit; and a rear lens unit, wherein the single focal length lens system does not include any other lens unit on the optical axis, and the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and an aperture stop is disposed on the image side of the second lens unit, and any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and the following conditional expression (1) is satisfied: 0.06<|f _(G3) /f|<0.4  (1) where, f_(G3) denotes a focal length of the third lens unit, and f denotes a focal length of the single focal length lens system at the time of focusing at an object at infinity.
 2. A single focal length lens system comprising in order from an object side to an image side along an optical axis: a front lens unit; and a rear lens unit, wherein the single focal length lens system does not include any other lens unit on the optical axis, and the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and an aperture stop is disposed on the image side of the second lens unit, and any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and the following conditional expression (2) is satisfied: 0.03<f _(b) /f<0.2  (2) where, f_(b) denotes a distance along the optical axis from an image-side surface of a lens nearest to an image, of the single focal length lens system up to an image plane, at the time of focusing at an object at infinity, and f denotes a focal length of the single focal length lens system at the time of focusing at an object at infinity.
 3. A single focal length lens system comprising in order from an object side to an image side along an optical axis: a front lens unit; and a rear lens unit, wherein the single focal length lens system does not include any other lens unit on the optical axis, and the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and an aperture stop is disposed on the image side of the second lens unit, and any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and the following conditional expressions (3) and (4) are satisfied: 1<ν_(G2nMAX)−ν_(G2pMIN)<40  (3) 37<ν_(G2nMAX)<55  (4) where, ν_(G2nMAX) denotes a maximum Abbe number from among Abbe numbers for negative lenses in the second lens unit, and ν_(G2pMIN) denotes a minimum Abbe number from among Abbe numbers for positive lenses in the second lens unit.
 4. A single focal length lens system comprising in order from an object side to an image side along an optical axis: a front lens unit; and a rear lens unit, wherein the single focal length lens system does not include any other lens unit on the optical axis, and the front lens unit includes in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a negative refractive power, and each of the first lens unit and the second lens unit includes a positive lens and a negative lens, and the first lens unit and the second lens unit do not move, and positions thereof are fixed all the time, and an aperture stop is disposed on the image side of the second lens unit, and any one of the lens units positioned on the image side of the second lens unit is a focusing lens unit, and the focusing lens unit moves along the optical axis at the time of focusing from an object at infinity to an object at a close distance, and the following conditional expression (5) is satisfied: 0.6<L _(TL) /f<0.88  (5) where, L_(TL) denotes a distance along the optical axis from an object-side surface of a lens nearest to the object, of the single focal length lens system up to an image plane, at the time of focusing at an object at infinity, and f denotes a focal length of the single focal length lens system at the time of focusing at an object at infinity.
 5. The single focal length lens system according to claim 1, wherein the third lens unit is the focusing lens unit.
 6. The single focal length lens system according to claim 1, wherein the third lens unit is the only focusing lens unit.
 7. The single focal length lens system according to claim 1, wherein the following conditional expression (6) is satisfied: 1.5<f _(G2) /f _(G3)<6.0  (6) where, f_(G2) denotes a focal length of the second lens unit, and f_(G3) denotes a focal length of the third lens unit.
 8. The single focal length lens system according to claim 1, wherein the following conditional expression (3) is satisfied: 1<ν_(G2nMAX)−ν_(G2pMIN)<40  (3) where, ν_(G2nMAX) denotes a maximum Abbe number from among Abbe numbers for negative lenses in the second lens unit, and ν_(G2pMIN) denotes a minimum Abbe number from among Abbe numbers for positive lenses in the second lens unit.
 9. The single focal length lens system according to claim 1, wherein the following conditional expression (7) is satisfied: −0.4<f _(fo) /f<−0.06  (7) where, f_(fo) denotes a focal length of the focusing lens unit, and f denotes the focal length of the single focal length lens system at the time of focusing at an object at infinity.
 10. The single focal length lens system according to claim 1, wherein the front lens unit has a positive refractive power as a whole.
 11. The single focal length lens system according to claim 1, wherein the focusing lens unit has a negative refractive power.
 12. The single focal length lens system according to claim 1, wherein the first lens unit includes a 1-1^(th) sub lens unit on the object side, and a 1-2^(th) sub lens unit on the image side, and an air space on an optical axis between the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit is the longest in the first lens unit, and each of the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit has a positive refractive power, and each of the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit includes a positive lens and a negative lens, and the following conditional expression (8) is satisfied: 0.35<D _(G1) /f _(G1)<1.3  (8) where, D_(m) denotes a distance on the optical axis from a lens surface nearest to an object of the first lens unit up to a lens surface nearest to an image of the first lens unit, and f_(G1) denotes the focal length of the first lens unit.
 13. The single focal length lens system according to claim 1, wherein the first lens unit includes a 1-1^(th) sub lens unit on the object side, and a 1-2^(th) sub lens unit on the image side, and an air space on an optical axis between the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit is the longest in the first lens unit, and each of the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit has a positive refractive power, and each of the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit includes a positive lens and a negative lens, and the following conditional expression (9) is satisfied: 0.1<D _(G112) /f _(G1)<0.6  (9) where, D_(G112) denotes a distance on the optical axis between the 1-1^(th) sub lens unit and the 1-2^(th) sub lens unit, and f_(m) denotes the focal length of the first lens unit.
 14. The single focal length lens system according to claim 1, wherein the rear lens unit includes an image-motion correcting lens unit, and the image-motion correcting lens unit moves in a direction different from a direction of the optical axis to reduce an image motion due to shaking of the single focal length lens system.
 15. The single focal length lens system according to claim 14, wherein the rear lens unit includes a first predetermined lens unit which is disposed immediately before the image-motion correcting lens unit, and the first predetermined lens unit has a refractive power having a sign different from a sign of a refractive power of the image-motion correcting lens unit.
 16. The single focal length lens system according to claim 14, wherein the rear lens unit includes a second predetermined lens unit which is disposed immediately after the image-motion correcting lens unit, and the second predetermined lens unit has a refractive power having a sign different from a sign of a refractive power of the image-motion correcting lens unit.
 17. The single focal length lens system according to claim 14, wherein the image-motion correcting lens unit includes a plurality of lenses and a predetermined lens, and the plurality of lenses have a refractive power having a sign same as the sign of the refractive power of the image-motion correcting lens unit, and the predetermined lens has a refractive power having a sign different from the sign of the refractive power of the image-motion correcting lens unit.
 18. The single focal length lens system according to claim 14, wherein the rear lens unit includes a first predetermined lens unit which is disposed immediately before the image-motion correcting lens unit and a second predetermined lens unit which is disposed immediately after the image-motion correcting lens unit, and each of the first predetermined lens unit and the second predetermined lens unit has a refractive power having a sign different from the sign of the refractive power of the image-motion correcting lens unit.
 19. The single focal length lens system according to claim 14, wherein the image-motion correcting lens unit has a negative refractive power.
 20. The single focal length lens system according to claim 1, wherein lens units included in the front lens unit are only the first lens unit, the second lens unit, and the third lens unit.
 21. The single focal length lens system according to claim 1, wherein the front lens unit includes in order from the object side to the image side, the first lens unit, the second lens unit, and the third lens unit, and the rear lens unit includes a lens unit having a positive refractive power, and the lens unit having a positive refractive power is disposed nearest to the object, in the rear lens unit, and a position of the lens unit having a positive refractive power is fixed, and the lens unit having a positive refractive power includes the aperture stop.
 22. The single focal length lens system according to claim 1, wherein only the focusing lens unit is a lens unit which is movable in the optical axial direction.
 23. The single focal length lens system according to claim 14, wherein only the focusing lens unit and the image-motion correcting lens unit are the lens units which are movable.
 24. The single focal length lens system according to claim 1, wherein the focusing lens unit includes maximum of two lenses.
 25. The single focal length lens system according to claim 1, wherein the focusing lens unit consists of two lenses, which are, one positive lens and one negative lens.
 26. The single focal length lens system according to claim 1, wherein the rear lens unit includes a positive lens and a negative lens.
 27. The single focal length lens system according to claim 1, wherein the rear lens unit includes in order from the object side to the image side, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power, and the lens unit having a negative refractive power in the rear lens unit is an image-motion correcting lens unit, and the image-motion correcting lens unit moves in a direction different from a direction of the optical axis, to reduce an image motion due to shaking of the single focal length lens system.
 28. The single focal length lens system according to claim 27, wherein the two lens units having a positive refractive power in the rear lens unit are disposed such that one lens unit having a positive refractive power is on the object side of the lens unit having a negative refractive power, immediately before the lens unit having a negative refractive power, and the other lens unit having a positive refractive power is on the image side of the lens unit having a negative refractive power, immediately after the lens unit having a negative refractive power, and both the lens units having a positive refractive power are lens units which do not move, and are fixed all the time.
 29. The single focal length lens system according to claim 1, wherein the single focal length lens system includes an image-motion correcting lens unit which satisfies the following conditional expression (10): 0.8<|MG _(ISback)×(MG _(IS)−1)|<5.0  (10) where, MG_(IS) denotes a lateral magnification of the image-motion correcting lens unit in an arbitrary focused state, and MG_(ISback) denotes a lateral magnification of an overall optical system between the image-motion correcting lens unit and the image plane, in an arbitrary focused state.
 30. The single focal length lens system according to claim 1, wherein the focusing lens unit satisfies the following conditional expression (11): 1.5<|(MG _(foback))²×{(MG _(fo))²−1}|<8.0  (11) where, MG_(fo) denotes a lateral magnification of the focusing lens unit in an arbitrary focused state, and MG_(foback) denotes a lateral magnification of the overall optical system between the focusing lens unit and the image plane, in an arbitrary focused state.
 31. The single focal length lens system according to claim 1, wherein the following conditional expression (12) is satisfied: 0.20<|f _(G1) /f _(G2)|<3.0  (12) where, f_(G1) denotes a focal length of the first lens unit, and f_(G2) denotes the focal length of the second lens unit.
 32. The single focal length lens system according to claim 1, wherein the aperture stop is disposed immediately after the second lens unit.
 33. The single focal length lens system according to claim 1, wherein the rear lens unit includes in order from the object side to the image side, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power, and the lens unit having a positive refractive power positioned on the object side of the lens unit having a negative refractive power in the rear lens unit includes two sub lens units having a positive refractive power, and the aperture stop is disposed to be sandwiched between the two sub lens units.
 34. The single focal length lens system according to claim 1, wherein the following conditional expression (14) is satisfied: 0≦|f/r _(G2b)|<7.0  (14) where, f denotes the focal length of the single focal length lens system at the time of focusing at an object at infinity, and r_(G2b) denotes a paraxial radius of curvature of a lens surface on the object side of the focusing lens unit, immediately before the focusing lens unit.
 35. The single focal length lens system according to claim 1, wherein the following conditional expression (15) is satisfied: 0.5≦φ_(fo)/φ_(La)≦0.92  (15) where, φ_(fo) denotes a maximum effective aperture from among effective apertures of lenses in the focusing lens unit, and φ_(La) denotes a maximum effective aperture of a lens positioned nearest to the image in the single focal length lens system.
 36. The single focal length lens system according to claim 1, wherein the following conditional expression (16) is satisfied: 0.023≦D _(sfo) /D _(LTL)≦0.110  (16) where, D_(sfo) denotes a distance on the optical axis from the aperture stop up to a lens surface nearest to an object of the focusing lens unit, D_(LTL) denotes a distance on the optical axis from a lens surface nearest to the object of the single focal length lens system up to an image plane, and both D_(sfo) and D_(LTL) are distances at the time of focusing at an object at infinity.
 37. The single focal length lens system according to claim 1, wherein the following conditional expression (17) is satisfied: 0.2≦D _(sfo)/φ_(s)≦0.8  (17) where, D_(sth) denotes the distance on the optical axis from the aperture stop up to a lens surface nearest to an object of the focusing lens unit, and is a distance at the time of focusing at an object at infinity, and (Φ_(s) denotes a maximum diameter of the aperture stop.
 38. The single focal length lens system according to claim 1, wherein an optical system positioned on the image side of the focusing lens unit includes at least two positive lenses and one negative lens.
 39. An image pickup apparatus comprising: an optical system; and an image pickup element which has an image pickup surface, and which converts an image formed on the image pickup surface by the optical system to an electric signal, wherein the optical system is a single focal length lens system according to claim
 1. 